DE 202 07 852.3 dated May 21, 2002 and DE 202 07 850.7 dated May 21, 2002
1. Field of the Invention
The present invention relates to an anchoring element for fastening a rod of a device for adjusting a human or animal spine to a vertebral bone, said anchoring element having a holding device that is substantially U-shaped and includes two substantially parallely disposed holding ridges, a rod receiving slot being formed therein, and a securing element acting against the rod accommodated in the receiving slot.
2. Description of the Prior Art
A pedicle screw the threaded shank of which is anchorable within a vertebral bone and which comprises at its end protruding from the vertebral bone a U-shaped holding device having two parallely oriented holding ridges is known from DE 94 02 839. Between the holding ridges there is formed a slot for receiving a distraction or compression rod that can be secured and fixed by means of a securing element which is fastenable to the free ends of the holding ridges. In a first embodiment, said securing element is composed of a hexagonal ring-shaped element having a locking screw embedded therein, the entire securing element being slidable into a corresponding groove provided in the holding ridges. The securing element straddles the receiving slot so that the locking screw acts directly onto the rod to fix it.
In another embodiment, the securing element is composed of a sleeve with internal thread surrounding the holding element, said sleeve being screwable onto a mating thread provided on the outer sides of the holding ridges so that the locking screw provided on the front side of the sleeve is allowed to engage into the receiving slot in order to fix the rod.
In such a device the pedicle screw must first be anchored within the vertebral body and then the rod is placed into the receiving slot. Next, and prior to bringing the vertebra or the rod into the desired position by tightening the locking screw, the securing element is either inserted into the grooves or screwed onto the holding device. It has been found that this is very difficult to carry out in practice because both inserting the securing element into the grooves and screwing the securing element onto the holding device require great dexterity, the vertebral body and/or the rod concurrently having to be prevented from coming out of place in the meantime.
An anchoring element having holding ridges on the inner sides of which there is provided a buttress thread for receiving the locking screw is known from DE 297 10 484. This feature permits to eliminate the forces which, in the case of a normal thread, act radially onto the holding ridges so that only axially acting forces are applied onto the holding ridge. As a result, the holding ridges are prevented from being bent apart so that the ring element surrounding the holding ridges may also be absent. In this case also, it has been found very tedious to screw the respective one of the locking screws into the corresponding buttress thread while maintaining the spine and/or the rod in the desired position until the locking screw takes hold of the rod by itself.
The term buttress thread is meant to include, besides the buttress metric thread DIN 513, buttress threads having a slightly larger or slightly smaller flank angle, a flank angle of 0° or a negative flank angle as well as buttress threads in accordance with EP 885 598.
An anchoring element also comprising a pedicle screw with U-shaped holding ridges integrally formed therewith is known from WO 95/01132. Pins directed toward the inside of the U-shape are formed integrally with said holding ridges. Said anchoring element further includes a flange-shaped securing element on the top side of which there is formed a protruding circumferential collar and on the bottom side of which there are provided two angled grooves for inserting said securing element into the pins of the holding elements. The securing element further includes an internal thread into which a grub screw is screwable for fixing the rod.
During surgery, the pedicle screw is first inserted into the corresponding site in the bone before the fixation rod is caused to enter the U-shaped holding device. Due to the little space available within the U-shaped holding device, the pin can only be designed very small on the inner side of the holding ridges. For this reason, it is very difficult for the surgeon, upon insertion of the fixation rod, to place with aiming accuracy the securing element onto said pins and to pivot it. This operation is rendered even more difficult by the fact that the surgeon cannot see the pin, which is concealed, while inserting the securing element into the U-shaped holding device. In this case as well, the fixation rod is first roughly held by the securing element prior to being fixed in its final position by the grub screw.
A pedicle screw the threaded shank of which is anchorable within a vertebral bone and which comprises at its end protruding from the vertebral bone a U-shaped holding device having two parallely oriented holding ridges is known from DE 92 02 745 U1. Between the holding ridges there is formed a slot for receiving a distraction or compression rod that can be secured and fixed by means of a securing element which is fastenable to the free ends of the holding ridges. Said securing element is configured to be a grub screw and engages in corresponding thread portions disposed on the respective one of the internal sides of the holding ridges. For actuating the grub screw, a recess for receiving a hexagonal wrench is formed at its upper front side.
U.S. Pat. No. 6,224,598 B1 discloses an anchoring element for fastening a rod to a spine that is also composed of a pedicle screw, a U-shaped holding device integrally formed therewith and a locking element. Here, the locking element is made of two parts, an outer part being configured to be a sleeve having both an internal and an external thread and an inner part being formed like a bolt and comprising a thread on its one end and a hexagon nut on the other end. The sleeve is thereby screwed by hand into the thread of the holding element, temporarily retaining the fixation rod. Next, the bolt is screwed into the sleeve until the thread of the bolt has completely disappeared in the sleeve. Upon screwing the bolt further down, it carries the sleeve along and both parts are moved toward the fixation rod to reliably make it fast.
In such type devices, the pedicle screw must first be screwed into the vertebral body and then the rod may be placed into the receiving slot. The grub screw is then screwed into the holding device to loosely retain the rod. Next, the vertebral body is brought into the desired position by setting the rod before the grub screw is firmly tightened to fix the rod in this position. The rod may thereby be configured as a distraction, a compression or a connection rod for example.